1. Field of the Technology
The present technology relates to a manufacturing method for a variable resistive element, which is made of a variable resistor provided between a first electrode and a second electrode, and of which the electrical resistance can be made to vary by applying a voltage pulse between the two electrodes.
2. Description of the Related Art
In recent years, a variety of device structures such as FeRAM's (Ferroelectric RAM's), MRAM's (Magnetic RAM's) and OUM's (Ovonic Unified Memories) have been proposed as a next generation nonvolatile random access memory (NVRAM) which can replace flash memory and can operate at a high speed, and are involved in severe competition for development in regards to enhancement in the performance, an increase in the reliability, reduction in cost and adjustment in the process. However, these memory devices at present respectively have advantages and disadvantages and an ideal “universal memory” having all the advantages of SRAM, DRAM and flash memory is still far from being implemented.
In comparison with these state of the art memory devices, a method for changing the electrical resistance in a reversible manner by applying a voltage pulse to a perovskite material, which is known as a colossal magnetoresistance effect, has been disclosed in the specification of U.S. Pat. No. 6,204,139 by Shangquing Liu, Alex Ignatiev et al. of Houston University in the U.S. and in “Electric-pulse-induced reversible resistance change effect in magnetoresistive films,” Applied Physics Letter, Vol. 76, by Liu, S. Q. et al. pp. 2749-2751 in 2000. This technology is extremely groundbreaking in that it uses a perovskite material, which is known to have a colossal magnetoresistance effect and where a change in the resistance over several digits is exhibited at room temperature without applying a magnetic field. The structure of the variable resistive element which is shown in the specification of U.S. Pat. No. 6,204,139 has a lower electrode material that is made of a yttrium barium copper oxide, YBa2Cu3O7 (YBCO), film which is deposited on a single crystal substrate made of a lanthanum aluminum oxide, LaAlO3 (LAO), a variable resistor film that is made of a perovskite-type oxide made of a crystal praseodymium calcium manganese oxide, Pr1-XCaXMnO3 (PCMO), film and an upper electrode material made of an Ag film which is deposited by means of sputtering. In addition, it has been reported on the operation of this variable resistive element that the resistance can be changed in a reversible manner by applying a voltage pulse of plus or minus 51 volts between the upper and lower electrodes. A resistance random access memory (RRAM) which uses the operation of the reversible change in the resistance of this variable resistive element (hereinafter appropriately referred to as “switching operation”) requires no magnetic field unlike MRAM, and therefore, has excellent properties such that power consumption is extremely low, miniaturization and increase in the integration are easy, and the dynamic range of the change in the resistance is significantly broad in comparison with MRAM, making multilevel storage possible.
In addition, as for the materials for a variable resistor, a ZnSe—Ge heterostructure and oxides of metals such as Ti, Nb, Hf, Zr, Ta, Ni, V, Zn, Sn, In, Th, and Al in addition to the above described perovskite materials are known as the materials which become a semiconductor when the composition ratio is off from the stoichiometric composition ratio, and of which the resistance value is variable depending on the applied voltage pulse conditions though it is at a small level.
An example of a conventional manufacturing method for this variable resistive element is described in the following.
FIG. 12 is a cross sectional diagram showing the basic structure of a variable resistive element. In addition, FIG. 13 is a flow chart showing the schematic manufacturing steps in accordance with a conventional manufacturing method for fabricating the variable resistive element.
This variable resistive element has a structure where a second electrode 1, which becomes a lower electrode, a variable resistor 2 and a first electrode 3 which becomes an upper electrode are layered sequentially in the direction perpendicular to the substrate. In addition, metal wires 6 are provided through contact holes 5 which are created in an interlayer insulating film 4 in order to apply a voltage pulse between the first electrode 3 and the second electrode 1.
The present Applicants fabricated a variable resistive element using, as an example of the variable resistor 2, a PCMO film which is an oxide having a perovskite-type structure and was formed so as to have a composition ratio of Pr0.7Ca0.3MnO3 and a film thickness of 100 nm at a temperature for film formation of 500° C., and evaluated the properties thereof. FIG. 14 is a graph showing the change in the resistance value when a voltage pulse having the positive polarity (+2 V of the first electrode and 0 V of the second electrode) and the negative polarity (0 V of the first electrode and +2 V of the second electrode) with a pulse width of 100 nsec is alternately applied to this variable resistive element. The lateral axis indicates the number of applied pulses and the longitudinal axis indicates the resistance value that is read out along the logarithmic scale. As for the number of applied pulses, alternate applications of a voltage pulse having a negative polarity and a voltage pulse having a positive polarity are counted as one pulse. As shown in FIG. 14, a repeating switching operation was confirmed when the resistance value in a low resistance state was approximately 102Ω and the resistance value in a high resistance state was approximately 104Ω. In addition, the ratio of the resistance value in the high resistance state to the resistance value in the low resistance state (hereinafter appropriately referred to as “switching ratio”) is approximately 100, and thus the two resistance values are clearly distinguished.
In the case where this variable resistive element is applied to a resistance nonvolatile memory, there is a restriction in the resistance value of the variable resistive element, in particular, in the resistance value of the variable resistive element in the low resistance state as described in the following.
The data in the nonvolatile memory is determined by the level of the amount of current which flows through the variable resistive element that is selected at the time of readout (hereinafter appropriately referred to as “readout current”). In the case of the simplest binary memory, for example, the amount of the read out current differs between when the variable resistive element is in the low resistance state and in the high resistance state, and therefore, the voltage value that has been converted from the readout current of the selected variable resistive element and the reference voltage are compared using a sense amplifier circuit, and thereby, each piece of data is determined as a binary value “1” or “0.” Here, in the case where the resistance value of the variable resistive element in the low resistance state is too high, the difference in the amount of the readout current between the time of the low resistance state and the high resistance state becomes small even when the switching ratio is sufficiently large, and therefore, the margin of the performance of the sense amplifier circuit becomes small and a problem arises where the data can not be correctly determined.
Meanwhile, in the case where the resistance of the variable resistive element is too low, the problem described below arises.
In a resistance nonvolatile memory using variable resistive elements, a number of variable resistive elements are connected to wires (so-called word lines and bit lines) for selecting a variable resistive element as a memory cell. When the resistance value of a variable resistive element becomes low, the amount of current that flows through an unselected variable resistive element other than the variable resistive element which is the selected memory cell increases in accordance with this low resistance, and therefore, the total amount of current which flows through the wires increases due to this extra current which flows through unselected variable resistive elements. When this total amount of current increases, the voltage drop along the wires increases, and therefore, in the particular case where the variable resistive element which is the selected memory cell is located at the end of the wires, the supplied voltage becomes insufficient and a problem arises where normal writing and readout operations can not be carried out.
Accordingly, the resistance value of the variable resistive element must be set within a predetermined range, in order to make the resistance nonvolatile memory operate normally. That is to say, the upper limit of the resistance value is limited by the performance of the sense amplifier which is determined by the configuration of the sense amplifier circuit, and the lower limit of the resistance value is determined by the number of cells in the variable resistive elements, which are memory cells connected to word lines and bit lines, the wire resistance of these lines and the current supplying performance of bank selecting transistors in the peripheral circuit connected to these lines.
Here, a case is assumed where a variable resistive element is applied to a certain example of a resistance nonvolatile memory (hereinafter, this nonvolatile memory is referred to as “nonvolatile memory A”) which operates normally when the resistance value of the variable resistive element in the low resistance state is in a range from 103Ω to 105Ω, and where it is more desirable for the resistance value to be approximately 104Ω, taking inconsistencies in the reproducibility of the resistance value when the nonvolatile memory is fabricated into consideration. The range of resistance is determined by the specifications of design for each nonvolatile memory, as described above (in the nonvolatile memory A of the present example, the number of memory cells which are connected to the word lines and the bit lines is 128, and the peripheral circuit which includes a sense amplifier and bank selecting transistors is designed in accordance with a design rule of 0.25 μm).
In the above described conventional manufacturing method for a variable resistive element, however, the resistance value of the variable resistive element in the low resistance state becomes approximately 102 Ω, which is too low, and therefore, a sufficient readout voltage cannot be supplied to some selected cells, due to the voltage drop in the lines to which a number of variable resistive elements are connected, and thus, the data cannot be correctly read out.
As for the control means for raising a resistance vale of the variable resistive element which is too low to a desired range in the conventional manufacturing method, though there are methods such as reducing the area of the variable resistive element or increasing the film thickness of the variable resistor, any method can contribute to increasing the resistance value in a linear function, and therefore, it is difficult to increase the resistance value by approximately 1 to 2 digits as required in the present example.
In addition, though the resistance value of the variable resistive element can be increased by changing the conditions for the formation of a variable resistor film, for example, the material composition or the temperature for film formation, a sufficient switching ratio for separating the high resistance state and the low resistance state must be secured in addition to control over the resistance value, and therefore, optimization of these conditions is not easy.
Therefore, in view of the above described problems, an objects of the present invention is to provide a manufacturing method where the resistance value in the low resistance state of the variable resistive element can be controlled. In particular, an object of the invention to provide a manufacturing method where the resistance value of the variable resistive element, which is too low in accordance with the conventional manufacturing method, can be controlled to a desired resistance value without causing the switching ratio to decline.